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Abstract

Multifunctional SiO2 · Re2O3 (Re = Y, Eu, La, Sm, Tb, Pr) hollow spheres (HSs) have been fabricated using an acidic
Re3+ ion solution. Under ultraviolet radiation, functional HSs emit different colors of
light according to the different rare-earth ions embedded into the shell of SiO2 hollow spheres. The as-prepared hollow capsules were characterized by X-ray diffraction,
transmission electron microscopy, high-resolution transmission electron microscopy,
Brunauer-Emmett-Teller method, scanning electron microscopy, and energy-dispersive
spectrometry. Drug loading and release experiments have been carried out using SiO2 · Eu2O3 HSs that acted as drug carriers. The results demonstrate that the multifunctional
HSs exhibit a high storage capacity and the ability of retaining drug stability and
activity, which indicates that the as-synthesized fluorescent hollow capsules are
a potential candidate as drug delivery materials.

Keywords:

SiO2 · Re2O3; Hollow spheres (HSs); Fluorescence

Background

Recently, much attention have been focused on the research of hollow SiO2 spheres (HSSs) because of their excellent properties such as thermal stability, large
surface areas, low density, low toxicity, and good compatibilities with other materials
[1-15]. So, HSSs have attracted intense interest and have been widely applied in a variety
of fields, such as catalysis, sensors, chromatography, dyes, inks, photonic crystals,
cells, waste removal, shield for enzymes or proteins, delivery vehicle of drugs, and
large biomolecular release [16-28]. In general, three approaches are employed to prepare HSSs: template methods [6,29-31], self-assembly technique, and microemulsions [32,33]. HSSs [34-36] have been fabricated with the soft template and hard template methods, which involve
complicated procedures such as shell formation and core removal. The template-free
method has attracted much attention due to its simple and economical characteristic
[27,28]. In 2008, Zhang et al. [25] developed a self-template method to convert solid silica into hollow spheres (HSs).
In the process, silica is dissolved into NaBH4 solution and silicate species deposited on the surface of the silica colloid. The
shell formed over the silicate species via Ostwald ripening results in HSSs. An alkalescent
environment is an inevitable synthesis condition that is reported in nearly all papers
[37-50]; the only exception is that of Chen et al. who used HF as an etching agent [51]. In 2011, Wang’s group reported firstly the synthesis of HSSs in generic acidic media
[52]. Unfortunately, the published work was based on the study of only two factors: pH
value and Na+ ion, which influence the formation process of HSSs.

Herein, we report a one-step self-driven process to synthesize multifunctional HSSs
under an acidic condition with rare-earth ion assistance. Compared with Wang’s report,
the synthetic approach of HSSs is simpler. Being synthesized with the assistance of
rare-earth ions, the as-prepared HSSs can emit bright fluorescence under ultraviolet
radiation, which is convenient to be detected in real time if it is used in biological
applications. Typical drug loading and release experiments are carried out using our
prepared multifunctional HSSs, SiO2 · Eu2O3 HSs.

Methods

All chemicals were of analytical grade and purchased from Jinan Camolai Trading Company
(Jinan, China), which were used as received without further purification: tetraethyl
orthosilicate (TEOS, 98%), ammonium hydroxide solution (NH3 · H2O, approximately 25% in water), nitric acid (HNO3, 65%), Re2O3, and ethanol (C2H5OH). Rare-earth nitrate solutions [Re(NO3)3 (Re = Y, Eu, La, Sm, Tb, Pr)] with a concentration of 0.04 to 0.08 mol/L were prepared
by ourselves.

Synthesis of monodisperse silica spheres

Silica spheres with a diameter of 200 to 500 nm were prepared by the hydrolysis of
TEOS in the mixture of ethanol, water, and ammonium using the Stöber process [37-39].

Synthesis of SiO2 · Re2O3 hollow spheres

In a typical synthesis, silica spheres (0.06 g) were added to 20 mL Re(NO3)3 (0.06 mol/L) and stirred for 30 min. The pH of the solution is 4.5 (adjusted with
dilute nitric acid). The mixture was transferred into a Teflon-lined stainless autoclave
(capacity 25 mL) and heated at 250°C for 12 h. After the products naturally cooled
down to room temperature, they were washed with deionized water and separated by centrifugation
(4,000 rpm) for three times and then dried at 60°C for 4 h in the air.

Drug storage and release

The steps of drug storage and release are as follows:

1. SiO2 · Eu2O3 HSs (1 g) were added into a 50-mL hexane solution containing 40 mg/mL ibuprofen (IBU).
The mixture was sealed and stirred for 24 h. Then the sample was separated by centrifugation
and dried at 60°C in the air. The filter was characterized by UV-visible (UV–vis;
264 nm) spectroscopy.

2. The dry SiO2 · Eu2O3 loaded with IBU (0.1 g) was immersed into 50 mL of simulated body fluid (SBF; pH = 7.4)
at 37°C and stirred at the rate of 100 rpm. Three milliliters from the top of the
solution was used for release measurement at different intervals, and then 3 mL of
fresh SBF is added into the solution to keep the volume unchanged.

Characterization and instruments

The characterization and instruments used are detailed as follows:

1. The samples were characterized by X-ray diffraction (XRD) with a Philips X’Pert
Super diffractometer (Amsterdam, The Netherlands) with graphite-monochromatized Cu
Kα radiation (λ = 1.54178 Å) in the 2θ range of 1.5° to 10° and 10° to 80°.

The samples used for characterization were ultrasonically dispersed in absolute ethanol
for 30 min before the TEM and HRTEM tests.

Results and discussion

Characterization of SiO2 · Eu2O3 HSs

Newly prepared silica spheres were used to fabricate HSSs. The monodispersed SiO2 spheres with an average diameter of 230 nm (Figure 1A) were fabricated using the Stöber method [37-39] and acted as the template. The hollow SiO2 · Eu2O3 HSs were uniform, as shown in the HRTEM image in Figure 1B, whose size was nearly unchanged. XRD curves in Figure 1C demonstrate that both the SiO2 sphere and SiO2 · Eu2O3 hollow sphere are amorphous (compared with ICSD #174). The absence of diffraction
peaks for Eu2O3 was owing to the few content of Eu2O3 in the sample. Figure 2 shows the HRTEM image and energy-dispersive spectrometer (EDS) analysis of SiO2 · Eu2O3 HSs. A large number of holes with different sizes on the surface of SiO2 · Eu2O3 HSs could be observed in Figure 2A, which belonged to a range of mesoporous structures according to the diameter of
holes. The SiO2 · Eu2O3 HSs with numerous mesoporous structures indicated that they are potential drug carriers
for application in medicine, e.g., targeting therapy. The results of the EDS analysis
showed that the content of O, Si, and Eu was 72.43%, 25.15%, and 2.22%, respectively.
The microcontent of Ge (0.19%) was due to the impurity coming from the reagent of
Eu2O3. The SiO2 HSs were amorphous according to their XRD pattern, so the lattice fringe that appeared
on the HRTEM image (Figure 2B) stemmed from Eu2O3. The measured interplanar spacing of 0.3 nm corresponded to the (001) plane of Eu2O3. Obviously, Eu2O3 is one component of the final product, and it may be embedded into the shells or
form a kind of composite similar to ‘alloy’ or a solid solution. Further research
is in progress. Being doped with Eu2O3 on the surface of SiO2 HSs, the obtained samples can emit bright red light under an ultraviolet beam. HRTEM
observation also revealed that the HSs produced in the solution contained Re3+ ions that formed a mesoporous structure with different orientations.

The measured emission spectrum curve of SiO2 · Eu2O3 HSs is shown in Figure 3. The strong red emission peak further suggested that Eu3+ existed in the surface of the SiO2 hollow sphere. The emission spectrum of SiO2 · Eu2O3 HSs consisted of peaks mainly located in the wavelength range from 570 to 700 nm.
These peaks corresponded to transitions from the excited state 5D0 to the ground state 7FJ (J = 0, 1, 2, 3, 4) of the 4f6 configuration of Eu3+, as marked in Figure 3. Luminescence originating from transitions between 4f levels is predominant due to electric dipole or magnetic dipole interactions [40-44]. As can be seen in Figure 3, the strong red emission peak at 612 nm originating from the electric dipole transition
5D0 to 7F2 was the dominant band in the measured spectrum. The emission peak at around 590 nm
was attributed to the 5D0 to 7F1 transition. The peaks located at 648 and 695 nm corresponded to 5D0 to 7F3 and 5D0 to 7F4 transitions, respectively.

Figure 4 shows the Brunauer-Emmett-Teller (BET) N2 adsorption-desorption isotherms and the pore size distribution of the as-prepared
SiO2 · Eu2O3 HSs. The BET specific surface area and the total pore volume of the SiO2 · Eu2O3 HSs were measured to be 308.6 m2/g and 0.307 cm3/g, respectively. The pore diameter distribution was relatively wide according to
the data of the adsorption branch of the isotherm. The as-prepared SiO2 · Eu2O3 HSs with a mesoporous structure may possess good performance in drug delivery efficiency,
catalytic activity, and so on.

The experiments showed that the pH value of the solution, reaction temperature and
time, and different rare-earth ions and concentrations played an outstanding role
in the synthesis of SiO2 · Re2O3 hollow structures, which are discussed in detail as follows.

Effect of the pH value of the solution

The pH value of the solution was adjusted with dilute nitric acid. The studied pH
range was from 7 to 3 under the following reaction conditions: Re3+ = 0.06 mol/L and T = 250°C. Hollow-structure particles could be obtained under the range of 4 ≤ pH < 5.5,
and the optimum pH value was 4.5. No hollow structure products appeared when 6 ≤ pH ≤ 8.
No HSSs appeared when 2 < pH < 3. Normally, a few HSSs could have emerged in the product
at the conditions of 3 < pH < 4.0 or 5 < pH < 6 if the reaction time was more than
10 h. The detailed results are shown in Additional file 1: Table S1 and Figure S3. It is known that SiO2 is an amphoteric oxide which can dissolve into an acidic or basic solvent. The experiments
showed that a weak acid solution was in favor of hollow structure formation. The experiments
also revealed that the experimental results were unaffected when using other acids
such as HCl or H2SO4 to adjust the pH value of the solution.

Effect of reaction temperature

The temperature of the hydrothermal reaction affected greatly not only the reaction
(going or not) but also the reaction rate (slow or fast). Additional file 1: Figure S1 shows the TEM images of the as-prepared samples at different reaction
temperatures. No hollow-structure products appeared if the temperature T < 230°C in our experiments. The morphology and size of nanocrystals became difficult
to control when the temperature was up to 260°C or higher because the higher the temperature
was, the faster the reaction rate was. When T = 255°C, the quality of the obtained SiO2 · Re2O3 HSs was always poor. The experiments verify that the moderate temperature was 250°C.

Effect of Re3+ ion and its concentration

It was reported that Na2SO4 and NaCl were advantageous to HSS formation [52] and the work matter was Na+ cation, which was in line with our experimental data. Hereby, we investigated the
synthesis of HSSs under different rare-earth ions and bivalent cations.

In order to get uniform hollow structures, the optimal concentration of the rare-earth
ions was usually kept in the range of 0.04 to 0.08 mol/L. The experimental data and
TEM images are depicted in Additional file 1: Table S1 and Figure S2. The concentration less than 0.03 mol/L resulted in poor
quality in production, and the concentration greater than 0.08 mol/L always led to
products with not all having a hollow structure. The experiments showed that the lower
or higher concentration of Re3+ ions was not good for HSS formation and 0.06 mol/L was the optimal concentration.

Although the SiO2 · Re2O3 HSs were obtained based on the rare-earth ion assistance strategy, their quality
was quite different under assistance of different kinds of rare-earth ions. By keeping
other reaction conditions unchanged such as the pH value of the solution, reaction
time, and reaction temperature, the influence of different Re3+ ions (Re = Y, Eu, La, Sm, Tb, Pr) on the structure of the as-prepared products was
investigated (see Additional file 1: Table S2 and Figure S4). Additional file 1: Figure S4 clearly shows that the influence sequence of Re3+ was as follows: Eu3 + ≈ Sm3 + > Y3 + > Pr3 + ≈ La3 + > Tb3 +. Nearly all of the as-prepared samples were hollow spheres with good quality under
the effect of Eu3+ and Sm3+ existence, and the experiments showed good reproducibility and satisfactory results.
With Y3+, Pr3+, and La3+ ions included, all of the products always formed a mixture of HSSs and core/shell
structure. Furthermore, all of the samples can be formed into a hollow sphere if the
reaction time is prolonged, but the yield of HSSs was lower. Only a small amount of
HSs could be obtained with Tb3+ existence. The experiments indicated that changing the reaction time did not work.

We compared the synthesis approach under the same reaction conditions using Mg2+, Ca2+, Mn2+, Zn2+, and Cu2+ instead of Re3+ ions. The results showed that SiO2 · HSs could barely be obtained at the above situations, which indicated that rare-earth
ion was an indispensable factor in hollow structure formation.

Experimental data showed that the rare-earth ions were advantageous to HSS formation;
however, further study is needed to understand why the effect of different Re3+ ions on the formation of HSSs has a different role.

Effect of reaction time

The reaction time will determine the deepening of the reaction after fixing other
reaction conditions. Figure 5 shows the structures of the as-prepared particles with a variety of reaction time.
As can be seen, rattle-type particles appeared after 6 h of reaction, and then the
core of particles gradually disappeared and finally became HSs at about 8 h, meanwhile
many tiny particles accompanied with the HSs. After 10 h, the shapes of HSs were clearer,
though many tiny particles were around them. The tiny particles came from the dissolved
SiO2, which disappeared with reaction time extension. The high-quality HSs with clear
edges were obtained when the reaction lasted for 12 h; simultaneously, the tiny particles
disappeared too. It was noticed that the shell of hollow spheres was getting thinner
and thinner when the reaction time was over 12 h. As can be seen, a handful of HSs
had cracked after 14 h. The experiments indicated that the reaction time would significantly
vary the influence on the shell thickness of SiO2 · Re2O3 HSs. Therefore, the shell thickness of HSs can be controlled by adjusting the reaction
time.

From the above, our synthesis procedure of HSSs is very simple and effective compared
with those previously reported.

Formation mechanism of SiO2 · Re2O3 HSs

In our experiments, SiO2 · Re2O3 HSs were synthesized in an acidic solution. It was reported that colloid SiO2 would carry a negative charge when pH > 3 [45]. The following equilibriums existed in the intermediate between the liquid and solid
interfaces [48]:

When Re3+ ions are added into the solution, an electrostatic force is produced between the
surface of silica and Re3+, Re3+ ions are absorbed onto the surface of SiO2 spheres at first, and then insoluble compounds SiO2 · Re2O3 are formed. Meanwhile, SiO2 cores keep dissolving in the acidic solution, as shown in Figure 5 (6 h). At the initial stage, most of the Re3+ ions are absorbed onto the surface of SiO2 spheres, and numerous insoluble tiny particles that come from the residual Re3+ ions meet with the negative ions in the solution, as shown in Figure 5 (8 and 10 h). As the reaction continues, the tiny particles are swallowed by the
SiO2 · Re2O3 lamella due to Ostwald ripening, and clear SiO2 · Re2O3 HSs are obtained at last, as shown in Figure 5 (12 h). Figure 6 is the sketch map of SiO2 · Re2O3 HS formation. In the first step, Re3+ would be adsorbed on the surface of the silica colloid, and then the silica colloid
turns into a rattle-type particle and finally turns into a HS through Ostwald ripening
after being treated in hydrothermal conditions.

The experiments showed that the diameter of SiO2 · Re2O3 HSs was almost the same as that of the template, which indicated that the size of
SiO2 · Re2O3 HSs was determined by the SiO2 solid spheres. Therefore, we can control the size of SiO2 · Re2O3 HSs by controlling the diameter of SiO2 solid spheres.

Drug delivery and release

Considering that HSs have numerous mesoporous structures on the surface, they can
act as drug loading capsules. IBU, a typical anti-inflammatory drug, is a good example
used for drug loading experiments [49,53]. Herein, IBU was used to study the drug delivery and release behavior of nanostructured
HSs.

The SiO2 · Re2O3 HSs were 1 g after loading IBU (see the ‘Methods’ section), and the IBU storage in
nanostructured SiO2 · Re2O3 HSs reached 287.8 mg/g, which means that the as-prepared SiO2 · Re2O3 HSs have a high loading capacity.

The rate of drug release determines the drug effect. Slow and sustained drug release
ensures a long drug effect. First of all, a phosphate buffer solution (PBS) of IBU
(0.1 μg/mL) was prepared to find out the maximum absorption wavelength using a UV-visible
spectrophotometer. The experiments indicated that the maximum absorption wavelength
of IBU was 264 nm. According to the Lambert-Beer law, A = kC, where A is the absorbency, k is a constant, and C is the concentration of IBU in PBS. The insert of Figure 7A is the working curve of IBU in PBS, which was obtained by the measured absorbency
of different PBS concentrations. The relationship between the concentration of IBU
in PBS and absorbency was as follows:

Figure 7.Release efficiency and UV–vis absorption spectra of IBU. (A) Release efficiency of IBU in the PBS system. The insert is the standard curve of
CIBU absorbance. (B) The UV–vis absorption spectra of IBU in the different release times. Curve a, IBU
hexane solution before drug loading; curve b, the SBF solution after the release of
IBU-loaded SiO2 · Eu2O3 HSs for 4 h; curve c, the SBF solution after the release of IBU-loaded SiO2 · Eu2O3 HSs for 70.

The released IBU concentration in SBF could be calculated using the following equation:

The total release rate of IBU can be calculated by the following equation:

where R is the total release rate, Ci is the IBU concentration in SBF at time i, i is the time of the IBU medium solution taking out from the SBF, and m represents the total mass of the IBU in the HSs.

Figure 7A shows the release behavior of the IBU-loaded SiO2 · Eu2O3 HSs in SBF. Compared with the pure IBU disk release in SBF, the release rate of the
IBU-loaded SiO2 · Eu2O3 HSs lasted long. The drug release rate was very fast within 12 h, which showed a
nearly linear relationship between drug release rate and release time at the first
12 h. Then the drug release process was very slow and finished after 60 h. The results
indicated that the sustained release behavior of the drug carrier was in favor of
a durative drug effect.

In order to investigate the properties of the loaded drug, the UV–vis absorption spectra
of the IBU hexane solution before and after IBU loading in SiO2 · Eu2O3 HSs were measured. The results are shown in Figure 7B. Curves a, b, and c were the IBU hexane solution before drug loading, the SBF solution
after the release of IBU-loaded SiO2 · Eu2O3 HSs for 4 h, and the SBF solution after the release of IBU-loaded SiO2 · Eu2O3 HSs for 70 h, respectively. It was noticed that the shape of the absorption curves
was essentially the same, which demonstrated that the property of IBU was not changed
in the loading and release processes.

We noticed that the samples still emitted fluorescence after the experiments of drug
delivery and release, which indicated that the leftover via the loading and release
processes can be tracked and detected.

Conclusions

We have reported an approach of the synthesis of functional SiO2 · Re2O3 HSs using silica spheres, rare-earth ions, and an acidic environment. The size of
synthesized hollow capsules can be modulated by controlling the diameter of the silica
template. The facile and economical synthesis protocol is valuable and convenient
for wide use. Acting as drug-loaded capsules, the SiO2 · Re2O3 HSs demonstrated much excellent properties of high payloads, retained drug activity
and stability, and slow drug release rate. Furthermore, real-time detection may be
carried out during drug delivery and release with SiO2 · Re2O3 HSs by measuring their fluorescence.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

ZL is the director of the experiment group, who propounded the ideas and drafted the
manuscript. LZ carried out the series of experiments and characterized all the samples.
QL participated in the related experiments. YD participated in the experiment of drug
loading and release. FW participated in its design and coordination. All authors read
and approved the final manuscript.

Acknowledgements

The authors express their thanks to Associate Prof. Rusen Yang (University of Minnesota)
for the language polishing.